Trichomonas vaginalis (Tv) accounts for nearly half of all curable sexually transmitted infections worldwide, including 5 million new infections annually in the US. Though infection is often asymptomatic in men and women, urethritis, vaginitis and cervicitis are not uncommon. In women, Tv has been associated with adverse birth outcomes, and infection is an important co-factor in HIV transmission. The extent of Tv strain variation and whether some strains are more likely to produce symptomatic or asymptomatic infection are not known. Reports of resistance to metronidazole have appeared and are increasing, but there have been few studies documenting the true extent of treatment failures, since distinction between reinfection and persistent infection await a practical typing system. Development of a reliable, molecular Tv typing system would improve understanding the epidemiology of Tv infection, resulting in better detection and treatment of trichomoniasis, which in turn are needed improve women's reproductive health and reduce HIV transmission where Tv is common. The specific aims are 1) To establish Tv genotypes based on sequence and length polymorphisms at sites in the parasite genome containing tandem DNA repeats. Loci containing variable numbers of tandem repeats will be amplified by PCR. Allele length polymorphisms will be exploited to generate a multi-locus typing system. 2) To define the genetic heterogeneity of Tv viruses (TVV) in recent clinical isolates to identify subgroups of TVV+ strains. Viral polymerase cDNA sequences will be generated and compared to reveal the relatedness and clustering patterns of TVVs. 3) To examine strain types of existing clinical specimens including Tv from sexual partners. Using the typing system developed in Aims 1 and 2, we will compare T. vaginalis strain types from infected women and their male partners attending two STD clinics in NC and one in AL to define the extent of strain differences within and between sexual partnerships and within and between clinic populations. We will also test the hypotheses that differences in Tv strain types contribute to clinical presentation and transmission by comparing the distribution of Tv types among women with or without symptoms and whose partners were or were not infected with Tv. The results of these studies may lead to the development of methods to identify particularly important Tv strains, improving treatment, and thereby reducing the adverse health consequences of this enormously prevalent STI.